Uncovering the chiral bias of meteoritic isovaline through asymmetric photochemistry

Systematic enrichments of l-amino acids in meteorites is a strong indication that biological homochirality originated beyond Earth. Although still unresolved, stellar UV circularly polarized light (CPL) is the leading hypothesis to have caused the symmetry breaking in space. This involves the differential absorption of left- and right-CPL, a phenomenon called circular dichroism, which enables chiral discrimination. Here we unveil coherent chiroptical spectra of thin films of isovaline enantiomers, the first step towards asymmetric photolysis experiments using a tunable laser set-up. As analogues to amino acids adsorbed on interstellar dust grains, CPL-helicity dependent enantiomeric excesses of up to 2% were generated in isotropic racemic films of isovaline. The low efficiency of chirality transfer from broadband CPL to isovaline could explain why its enantiomeric excess is not detected in the most pristine chondrites. Notwithstanding, small, yet consistent l-biases induced by stellar CPL would have been crucial for its amplification during aqueous alteration of meteorite parent bodies.

(1) where gL is the anisotropy factor corresponding to the L-enantiomer and depends on the wavelength of the circularly polarized light.

Supplementary Note 2: VUV/UV anisotropy spectroscopy of isotropic isovaline films
Absorption, circular dichroism and anisotropy data treatment The measurements of three independent thin films were chosen for the final data set for each of the two enantiomers based on their low level of scattering. Each set of measurements is represented by a thin dashed line in Supplementary Fig. 1a and b. These thin lines are averages of the absorption and CD spectra recorded for four different angles (0°, 90°, 180°, and 270°) defined by rotating the window holder around the axis of the incident synchrotron radiation normal to the sample surface. The level of agreement between the absorption and CD spectra recorded at these four angles for each sample allows us to exclude any significant effects due to linear birefringence and/or linear dichroism 3,4 . For better comparison between individual films of varying thicknesses, a normalization was applied to the CD spectra by scaling their corresponding absorbance in the wavelength range 155-180 nm. The final normalized absorption spectra are in Supplementary Fig. 1c and the normalized CD spectra are in Supplementary Fig. 1d. Note that the here applied normalization process has no influence on the anisotropy spectra, g = Δε/ε = CD / absorbance, as they are independent of any scaling factor applied to the absorbance and CD spectra. The resultant anisotropy spectra for both isovaline enantiomers are presented in Supplementary Fig. 2. Supplementary Fig. 1 Synchrotron radiation absorption and CD data. Absorbance and CD spectra of the L-(blue shading) and D-(red shading) enantiomers of isovaline: a and b before, and c and d after normalization, respectively. Thin dashed lines represent averaged data recorded at four different angles (0°, 90°, 180°, and 270°) for each film. Thick lines represent average over 3 films for the L-(blue) and D-(red) enantiomers. Source data are provided as a Source Data file.

Supplementary Fig. 2 Anisotropy spectra of the L-(blue shading) and D-(red shading) enantiomers of isovaline.
Thin dashed lines represent averaged data recorded at four different angles (0°, 90°, 180°, and 270°) for each film. Thick lines represent average over 3 films for the L-(blue) and D-(red) enantiomers.

Comparison of the present anisotropy spectra with previous data
The level of scattering we reached in the CD/anisotropy spectroscopy experiments on solid-phase L-and D-isovaline with the present sample preparation procedure was significantly reduced compared to our previous attempts of producing thin films of isovaline enantiomers using sublimation followed by deposition in a high vacuum sublimation-deposition chamber (Meinert et al., 2012 2 ). This is manifested by well-defined and quasi-perfectly mirrored anisotropy bands and zero-crossings of the two enantiomers in the present study ( Supplementary Fig. 3a) as opposed to 2012 2 . Even though the overall anisotropy signal (zero crossings, band signs, shapes, and intensities) from 2012 is significantly distorted, the green and purple bars in Supplementary Fig. 3a highlight the general agreement in the positions of the anisotropy bands in between the two data sets. To elucidate the effect of the sample preparation procedure (drop-casting vs sublimation-deposition) on sub-/microscopic film properties and hence their spectroscopic response, we performed scanning electron microscopy (SEM) imaging (Supplementary Information, S.1.3). Even though the exact same films could not be used for both SEM and CD/anisotropy spectroscopy, the selection of films for the SEM imaging was based on the similarity of their macroscopic appearance with the ones prepared in analogous conditions for the spectroscopy experiments. The L-isovaline film in Supplementary Fig. 4ac prepared by drop-casting is mostly amorphous with only sporadic nanocrystal nucleation sites, the presence of which could potentially explain the minor distortions in the CD and hence anisotropy spectra in the present study. Such nanocrystal nucleation sites are much more abundant in the ∼200 nm L-isovaline film prepared by sublimation-deposition ( Supplementary Fig. 4d-f) and turn to plate submicrocrystals with increasing film thickness ( Supplementary Fig. 4g-i). This is likely to explain the artefacts in the 2012 anisotropy spectrum in Supplementary Fig. 3a, where based on the absorbance ( Supplementary Fig. 3b), the film thickness was significantly larger compared to the present study. It is likely that the slow deposition of enantiopure gas-phase isovaline at elevated temperatures as opposed to fast methanol evaporation at moderate temperatures during drop-casting favours sub-/microcrystal growth. Clearly, the suite of isovaline structures and their relative abundance differs in the gas-phase (neutrals) and methanol solution (zwitterions). It is therefore possible that the former one exhibits higher predisposition to forming crystallisation nucleation sites, which are known to be able to further affect the conformations of crystallising species and hence facilitate the crystal growth 5 . On the contrary, the ∼400 nm film of racemic isovaline in Supplementary Fig. 4j-l exhibits non-crystalline isotropic amorphous character with no long-range order.

Scanning electron microscopy (SEM) imaging
SEM images of L-and DL-isovaline thin films prepared by drop-casting and/or sublimation-deposition techniques ( Supplementary Fig. 4) were recorded using JEOL JSM-6700F scanning electron microscope at Microscopie Imagerie Côte d'Azur. Prior SEM imaging, the CaF2 windows with the isovaline films were fixed using silver conductive paste to metallic support plates and coated with a 2 nm thick platinum layer. Supplementary Fig. 4 Distinct morphology of the L-and DL-isovaline films prepared by drop-casting and/or sublimation-deposition techniques. a-c SEM images of L-Isovaline film prepared by evaporation of 60 µL of 1 mg mL⁻ 1 L-isovaline standard solution in methanol at three different scales. A growing fan-like microcrystal in a surrounded by large amorphous areas with dispersed scarce nanocrystal nucleation sites are clearly visible in bc. d-f An ∼200 nm thick L-isovaline film prepared by sublimation of L-isovaline at 120 °C followed by deposition on a CaF2 window in the sublimation-deposition chamber (pressure ∼10 −5 mbar). The well distinguished grains in d represent unwanted dust particles fallen on the surface of the film. Numerous nanocrystal nucleation sites are visible in e-f. g-i An ∼300 nm thick L-isovaline film prepared by sublimation of L-isovaline at 133 °C followed by deposition on a CaF2 window in the sublimation-deposition chamber (pressure ∼10 −5 mbar) dominated by plate submicrocrystals. j-l The amorphous ∼400 nm thick racemic DL-isovaline film prepared by sublimation of DL-isovaline standard at 115 °C followed by deposition on a CaF2 window in the sublimation-deposition chamber (pressure ∼10 −7 mbar) is comprised of nanofiber networks exhibiting no long-range order. The white bars in the bottom right corners of the SEM images indicate a length of 1 µm.

Supplementary Note 3: Quantum chemical calculations
The initial molecular structures of isovaline were built using GaussView (version 6) 5 . Four distinct conformers Ia, IIa, IIb, and IIc were taken from previous work 6 . After converting each neutral structure to the zwitterionic form, theoretical calculations were carried out using time-dependent density functional theory (TDDFT) with the B3P86 hybrid functional in combination with the 6-311+G(d,p) basis set. In total, 200 excited states were calculated for each of the four conformers. The theoretical electronic circular dichroism spectra have been convoluted by means of summing rotatory strength weighted Gaussian distribution functions with full width at half maximum of 0.33 eV. All calculations were made using Gaussian 16 Rev. C.01. 7 . Among the four considered conformers, only conformer IIa provided a good agreement with the experimental CD spectrum (Supplementary Fig. 5). The energetically favored configuration according to the TDDFT calculation did not correlate most strongly with the experiment which is consistent with previous observations for isotropic films of alanine 8 . Supplementary Fig. 5 Comparison of the calculated and experimental CD spectra. The blue line accounts for the experimental CD spectrum of L-isovaline. The theoretical spectra were obtained using time-dependent density functional theory with the B3P86 hybrid functional in combination with the 6-311g+(d,p) basis set. Among the four most stable conformers, only IIa agrees qualitatively with the experimental spectrum (upper panel, green line). The lower panel displays the theoretical spectra obtained for the three conformers Ia, IIb, and IIc as brown, orange, and yellow line, respectively. Table 1